1. Technical Field
Methods and example implementations described herein are generally directed to hardware hash tables, and more specifically, to generation of a cache coherent Network on Chip (NoC).
2. Related Art
The number of components on a chip is rapidly growing due to increasing levels of integration, system complexity and shrinking transistor geometry. Complex System-on-Chips (SoCs) may involve a variety of components e.g., processor cores, DSPs, hardware accelerators, memory and I/O, while Chip Multi-Processors (CMPs) may involve a large number of homogenous processor cores, memory and I/O subsystems. In both SoC and CMP systems, the on-chip interconnect plays a role in providing high-performance communication between the various components. Due to scalability limitations of traditional buses and crossbar based interconnects, Network-on-Chip (NoC) has emerged as a paradigm to interconnect a large number of components on the chip. NoC is a global shared communication infrastructure made up of several routing nodes interconnected with each other using point-to-point physical links.
Messages are injected by the source and are routed from the source node to the destination over multiple intermediate nodes and physical links. The destination node then ejects the message and provides the message to the destination. For the remainder of this application, the terms ‘components’, ‘blocks’, ‘hosts’ or ‘cores’ will be used interchangeably to refer to the various system components, which are interconnected using a NoC. Terms ‘routers’ and ‘nodes’ will also be used interchangeably. Without loss of generalization, the system with multiple interconnected components will itself be referred to as a ‘multi-core system’.
There are several topologies in which the routers can connect to one another to create the system network. Bi-directional rings (as shown in FIG. 1(a)), 2-D (two dimensional) mesh (as shown in FIG. 1(b)) and 2-D Torus (as shown in FIG. 1(c)) are examples of topologies in the related art. Mesh and Torus can also be extended to 2.5-D (two and half dimensional) or 3-D (three dimensional) organizations. FIG. 1(d) shows a 3D mesh NoC, where there are three layers of 3×3 2D mesh NoC shown over each other. The NoC routers have up to two additional ports, one connecting to a router in the higher layer, and another connecting to a router in the lower layer. Router 111 in the middle layer of the example has both ports used, one connecting to the router at the top layer and another connecting to the router at the bottom layer. Routers 110 and 112 are at the bottom and top mesh layers respectively, therefore they have only the upper facing port 113 and the lower facing port 114 respectively connected.
Packets are message transport units for intercommunication between various components. Routing involves identifying a path composed of a set of routers and physical links of the network over which packets are sent from a source to a destination. Components are connected to one or multiple ports of one or multiple routers; with each such port having a unique ID. Packets carry the destination's router and port ID for use by the intermediate routers to route the packet to the destination component.
Examples of routing techniques include deterministic routing, which involves choosing the same path from A to B for every packet. This form of routing is independent from the state of the network and does not load balance across path diversities, which might exist in the underlying network. However, such deterministic routing may implemented in hardware, maintains packet ordering and may be rendered free of network level deadlocks. Shortest path routing may minimize the latency as such routing reduces the number of hops from the source to the destination. For this reason, the shortest path may also be the lowest power path for communication between the two components. Dimension-order routing is a form of deterministic shortest path routing in 2-D, 2.5-D, and 3-D mesh networks. In this routing scheme, messages are routed along each coordinates in a particular sequence until the message reaches the final destination. For example in a 3-D mesh network, one may first route along the X dimension until it reaches a router whose X-coordinate is equal to the X-coordinate of the destination router. Next, the message takes a turn and is routed in along Y dimension and finally takes another turn and moves along the Z dimension until the message reaches the final destination router. Dimension ordered routing may be minimal turn and shortest path routing.
FIG. 2(a) pictorially illustrates an example of XY routing in a two dimensional mesh. More specifically, FIG. 2(a) illustrates XY routing from node ‘34’ to node ‘00’. In the example of FIG. 2(a), each component is connected to only one port of one router. A packet is first routed over the x-axis till the packet reaches node ‘04’ where the x-coordinate of the node is the same as the x-coordinate of the destination node. The packet is next routed over the y-axis until the packet reaches the destination node.
In heterogeneous mesh topology in which one or more routers or one or more links are absent, dimension order routing may not be feasible between certain source and destination nodes, and alternative paths may have to be taken. The alternative paths may not be shortest or minimum turn.
Source routing and routing using tables are other routing options used in NoC. Adaptive routing can dynamically change the path taken between two points on the network based on the state of the network. This form of routing may be complex to analyze and implement.
A NoC interconnect may contain multiple physical networks. Over each physical network, there may exist multiple virtual networks, wherein different message types are transmitted over different virtual networks. In this case, at each physical link or channel, there are multiple virtual channels; each virtual channel may have dedicated buffers at both end points. In any given clock cycle, only one virtual channel can transmit data on the physical channel.
NoC interconnects may employ wormhole routing, wherein, a large message or packet is broken into small pieces known as flits (also referred to as flow control digits). The first flit is the header flit, which holds information about this packet's route and key message level info along with payload data and sets up the routing behavior for all subsequent flits associated with the message. Optionally, one or more body flits follows the head flit, containing the remaining payload of data. The final flit is the tail flit, which in addition to containing the last payload also performs some bookkeeping to close the connection for the message. In wormhole flow control, virtual channels are often implemented.
The physical channels are time sliced into a number of independent logical channels called virtual channels (VCs). VCs provide multiple independent paths to route packets, however they are time-multiplexed on the physical channels. A virtual channel holds the state needed to coordinate the handling of the flits of a packet over a channel. At a minimum, this state identifies the output channel of the current node for the next hop of the route and the state of the virtual channel (idle, waiting for resources, or active). The virtual channel may also include pointers to the flits of the packet that are buffered on the current node and the number of flit buffers available on the next node.
The term “wormhole” plays on the way messages are transmitted over the channels: the output port at the next router can be so short that received data can be translated in the head flit before the full message arrives. This allows the router to quickly set up the route upon arrival of the head flit and then opt out from the rest of the conversation. Since a message is transmitted flit by flit, the message may occupy several flit buffers along its path at different routers, creating a worm-like image.
Based upon the traffic between various end points, and the routes and physical networks that are used for various messages, different physical channels of the NoC interconnect may experience different levels of load and congestion. The capacity of various physical channels of a NoC interconnect is determined by the width of the channel (number of physical wires) and the clock frequency at which it is operating. Various channels of the NoC may operate at different clock frequencies, and various channels may have different widths based on the bandwidth requirement at the channel. The bandwidth requirement at a channel is determined by the flows that traverse over the channel and their bandwidth values. Flows traversing over various NoC channels are affected by the routes taken by various flows. In a mesh or Torus NoC, there may exist multiple route paths of equal length or number of hops between any pair of source and destination nodes. For example, in FIG. 2(b), in addition to the standard XY route between nodes 34 and 00, there are additional routes available, such as YX route 203 or a multi-turn route 202 that makes more than one turn from source to destination.
In a NoC with statically allocated routes for various traffic slows, the load at various channels may be controlled by intelligently selecting the routes for various flows. When a large number of traffic flows and substantial path diversity is present, routes can be chosen such that the load on all NoC channels is balanced nearly uniformly, thus avoiding a single point of bottleneck. Once routed, the NoC channel widths can be determined based on the bandwidth demands of flows on the channels. Unfortunately, channel widths cannot be arbitrarily large due to physical hardware design restrictions, such as timing or wiring congestion. There may be a limit on the maximum channel width, thereby putting a limit on the maximum bandwidth of any single NoC channel.
Additionally, wider physical channels may not help in achieving higher bandwidth if messages are short. For example, if a packet is a single flit packet with a 64-bit width, then no matter how wide a channel is, the channel will only be able to carry 64 bits per cycle of data if all packets over the channel are similar. Thus, a channel width is also limited by the message size in the NoC. Due to these limitations on the maximum NoC channel width, a channel may not have enough bandwidth in spite of balancing the routes.
To address the above bandwidth concern, multiple parallel physical NoCs may be used. Each NoC may be called a layer, thus creating a multi-layer NoC architecture. Hosts inject a message on a NoC layer; the message is then routed to the destination on the NoC layer, where it is delivered from the NoC layer to the host. Thus, each layer operates more or less independently from each other, and interactions between layers may only occur during the injection and ejection times. FIG. 3(a) illustrates a two layer NoC. Here the two NoC layers are shown adjacent to each other on the left and right, with the hosts connected to the NoC replicated in both left and right diagrams. A host is connected to two routers in this example—a router in the first layer shown as R1, and a router is the second layer shown as R2. In this example, the multi-layer NoC is different from the 3D NoC, i.e. multiple layers are on a single silicon die and are used to meet the high bandwidth demands of the communication between hosts on the same silicon die. Messages do not go from one layer to another. For purposes of clarity, the present application will utilize such a horizontal left and right illustration for multi-layer NoC to differentiate from the 3D NoCs, which are illustrated by drawing the NoCs vertically over each other.
In FIG. 3(b), a host connected to a router from each layer, R1 and R2 respectively, is illustrated. Each router is connected to other routers in its layer using directional ports 301, and is connected to the host using injection and ejection ports 302. A bridge-logic 303 may sit between the host and the two NoC layers to determine the NoC layer for an outgoing message and sends the message from host to the NoC layer, and also perform the arbitration and multiplexing between incoming messages from the two NoC layers and delivers them to the host.
In a multi-layer NoC, the number of layers needed may depend upon a number of factors such as the aggregate bandwidth requirement of all traffic flows in the system, the routes that are used by various flows, message size distribution, maximum channel width, etc. Once the number of NoC layers in NoC interconnect is determined in a design, different messages and traffic flows may be routed over different NoC layers. Additionally, one may design NoC interconnects such that different layers have different topologies in number of routers, channels and connectivity. The channels in different layers may have different widths based on the flows that traverse over the channel and their bandwidth requirements.
In a NoC interconnect, if the traffic profile is not uniform and there is certain amount of heterogeneity (e.g., certain hosts talk to each other more frequently than the others), the interconnect performance may depend a lot on the NoC topology and where various hosts are placed in the topology with respect to each other and to what routers they are connected to. For example, if two hosts talk to each other frequently and need higher bandwidth, they should be placed next to each other. This will reduce the latency for this communication, and thereby reduce the global average latency, as well as reduce the number of router nodes and links over which the high bandwidth of this communication must be provisioned. Moving two hosts close by may make certain other hosts far apart since all hosts must fit into the 2D planar NoC topology without overlapping with each other. Thus, right tradeoffs must be made and the hosts must be placed after examining the pair-wise bandwidth and latency requirements between all hosts so that certain global cost and performance metrics is optimized. The cost and performance metrics can include the average structural latency between all communicating hosts in number of router hops, or the sum of the bandwidth between all pair of hosts and the distance between them in number of hops, or some combination thereof. This optimization problem is known to be non-deterministic polynomial-time hard (NP-hard) and heuristic based approaches are often used. The hosts in a system may vary is shape and sizes with respect to each other which puts additional complexity in placing them in a 2D planar NoC topology, packing them optimally leaving little whitespaces, and avoiding overlapping hosts.
In a NoC architecture, there is a need to maintain cache coherency among the agents in the NoC as explained, for example, in U.S. patent application Ser. No. 13/965,668, herein incorporated by reference in its entirety for all purposes. Related art methods for maintaining cache coherency involve several methods for maintaining cache coherent data. In one example related art method, a single, universal copy of data is maintained, and the agents refer only to the universal copy. In another example related art method, the agents may include their own cache and thereby maintain their own copies of data. In a heterogeneous system wherein agents may or may not have their own cache, both of the related art methods can be employed and managed by using various cache coherency protocols such as MESI (Modified Exclusive Shared Invalid), MSI, MOESI (Modified Owned Exclusive Shared Invalid) and so on.
To manage the transition between states for the cache coherency protocols, related art methods perform a lookup of the coherent state on cache request. Based on the lookup of the coherent state, transition commands may be issued to existing caches to change their state. The lookup of the coherent state requires either a broadcasting to all of the agents associated with the NoC or a directory structure that tracks the current states and can be used for managing the states of the caches.
Snooping is a process where the individual caches monitor address lines for accesses to memory locations that they have cached instead of a centralized directory-like structure handling the monitoring. When a write operation is observed to a location that a cache has a copy of, the cache controller invalidates its own copy of the snooped memory location. In the snooping solution, a snoopy bus is incorporated to send all requests for data to all processors, wherein the processors snoop to see if they have a copy and respond accordingly. This mechanism therefore involves a broadcast, since caching information is stored in the processors. A multiple snoop filter reduces the snooping traffic by maintaining a plurality of entries, each representing a cache line that may be owned by one or more nodes. When replacement of one of the entries is required, the snoop filter selects for replacement the entry representing the cache line or lines owned by the fewest nodes, as determined from a presence vector in each of the entries. A temporal or other type of algorithm is used to refine the selection if more than one cache line is owned by the fewest number of nodes.
In related art implementations, there is a snooping architecture based on a directory structure. For every coherent lookup, a snoop is broadcast to the agents. If the agent has the data corresponding to the snoop, then the data is provided. If any agent has a dirty copy of the line, that agent is compelled to provide data in response to the snoop. However, in such configurations, the multiple coherent agents may experience high latency due to access to the memory controller through the coherency controller. Thus, if the copy can be obtained directly from the agent rather than retrieval through the memory controller, then latency can be reduced. In related art implementations, non-dirty agents can provide data directly to reduce the latency. However, because the agents are not coordinated, each agent will provide the copy autonomously.
In related art implementations for directory-based protocol, an address is tracked as well as the agents having a copy of the address. FIG. 4 illustrates an example directory. The directory may include entries for state (e.g. read only, read/write, etc.), address tag to indicate address within the cache containing the data, and a bit vector indicating agents that have caches containing or sharing the data. The bit vector is configured based on the specification. For example, if the NoC is associated with 64 agents, the bit vector may have a 64 bit long vector with each bit indicating if the data is held in the corresponding agent or not. In one aspect, the cache coherency controller can be configured to retrieve, from a directory, the state of the cache line and return the state of the cache line to a requesting memory communications controller.
However, the directory does not provide any indication as to which agent has a dirty copy of the line. In related art implementations, there is no preference system for selecting an agent that will provide data. Only a dirty agent is compelled to provide data, while the other agents may discard their own copy at any time. Such related art implementations attempt to resolve the bandwidth issue in several ways. One related art implementation involves having a guaranteed provider of data that is known. A protocol is constructed such that if the agent has the copy of the line, the agent is configured to permanently hold the copy until copy is acknowledged. Certain agents in the system will not have such a guarantee. Any agent in the exclusive or modified state would be required to provide data (E/M/O in MOESI, E/M in MESI).
In another related art implementation, a snoop operation is sent to the dirty agent. In a protocol such as MOESI, agents designated as M/O may contain dirty data, however this is not verifiable because the agent is silent. For example, an agent with ‘O’ status may downgrade its dirty copy. Although the data is assumed to be dirty, the dirtiness status is not communicated via the snoop.
In another related art implementation, all of the agents may be snooped to determine the agent with the dirty copy, however, such an implementation is bandwidth intensive.